The vanilloid receptor (VR1)-mediated effects of anandamide are potently enhanced by the cAMP-dependent protein kinase

The endogenous cannabinoid receptor ligand, anandamide (AEA), is a full ago nist of the vanilloid receptor type 1 (VR1) for capsaicin. Here; we demonst rate that the potency and efficacy of AEA at VR1 receptors can be significa ntly increased by the concomitant activation of protein kinase A (PKA). In human embryonic kidney (HEK) cells over-expressing human VR1, AEA induces a rise in cytosolic Ca2+ concentration that is mediated by this receptor. Th e EC50 for this effect was decreased five-fold in the presence of forskolin (FRSK, 1-5 muM) or the cAMP analogue, 8-Br-cAMP (10-100 muM). The effects of 8-Br-cAMP and FRSK were blocked by a selective PKA inhibitor. The FRSK(1 0 nM) also potently enhanced the sensory neurone- and VR1-mediated constric tion by AEA of isolated guinea-pig bronchi, and this effect was abolished b y a PKA inhibitor. In rat dorsal root ganglia slices, AEA-induced release o f substance P, an effect mediated by VR1 activation, was enhanced three-fol d by FRSK (10 nM). Thus, the ability of AEA to stimulate sensory VR1, with subsequent neuropeptide release, appears to be regulated by the state of ac tivation of PKA. This observation supports the hypothesis that endogenous A EA might stimulate VR1 under certain pathophysiological conditions.